A fiber optic gyroscope measures the rotation of a sensing coil by detecting the Sagnac phase shift between two counter-propagating light waves in the sensing coil.
The mounting of the sensing coil in a fiber optic gyroscope is critical for a number of reasons. Because the gyroscope must be capable of sensing extremely small movements in the coil, the coil should be mounted in a way that isolates the coil from stresses that might cause localized disturbances in the fiber which forms the coil. Such localized disturbances can modify the light wave in a manner that can be mistaken for rotation of the coil. The coil must also remain stable in a fixed position so as to avoid the generation of spurious signals due to movement of the coil relative to the structure on which it is mounted. Moreover, the isolation and fixed position of the coil must usually be maintained over a relatively wide temperature range, e.g., from -55.degree. C. to +85.degree. C.
In interferometric and resonant fiber optic gyroscopes, optical power is injected into both ends of the coil by a directional coupler, and exits the coil through the same directional coupler. The Sagnac ring (coil) in these gyroscopes is often made of single-mode, polarization-maintaining fiber, to ensure that light energy propagates along the length of the coil in the same polarization that it originally had at the directional coupler interface. Polarization maintenance is characterized by the h-parameter.
There also exist Sagnac rings made of single-mode (non-polarization maintaining) fiber where the optical power is de-polarized, typically by a Lyott depolarizer. Since the depolarization is often imperfect, changes in the birefringence of the coil fiber and movement of the coil with respect to its mounting can result in effects analogous to those described for polarization-maintaining fiber coils.
As is well-known in the art, the extinction ratio (ER) of a sensing coil is a measure of the polarization-holding properties of the coil. More specifically, the value of ER is a measure of the number of dB's between the intensity of an optical signal having the desired polarization and the intensity of an optical signal having polarization that is orthogonal to the desired polarization, after the desired optical signal has been propagated through a given length of optical fiber. The extinction ratio of a coil of length l is related to the h-parameter of the fiber, where h is defined as the extinction ratio per meter of fiber.
Thus EQU ER=10 log.sub.10 h+10 log.sub.10 l, dB
The h-parameter of a fiber is usually measured by determining the extinction ratio of a sample length of fiber, perhaps 100 meters, wound loosely on a large diameter form. Thus the h-parameter can be defined as ##EQU1## where l.sub.test test is the length of the test sample and P.sub.min and P.sub.max are the optical powers measured through the fiber with crossed polarizers and aligned polarizers, respectively. Thus P.sub.max represents the wanted signal level, and P.sub.min represents the unwanted (cross-polarized) signal level. Alternatively, the h-parameter is expressed in dB-meters, and is defined as EQU H=10 log.sub.10 h
A typical value of H is -45 dB-m.
The value of the h-parameter is a function of a number of factors such as the fiber construction, the protective (buffer) coating applied over the fiber, and external stresses. When the fiber is wound into a coil, the resultant extinction ratio depends on the size and winding of the coil and the method of mounting the coil in the gyroscope. The performance can be described in terms of ER, but it is perhaps better to use h, since coils of differing lengths and diameters are used to achieve specific product specifications. A poor h-parameter value (closer to 1.0) is manifested as gyro drift, which arises from unwanted coupling between the two polarization modes. This coupling is typically random in nature, but can have temperature-dependent effects.
Degradation of the h-parameter is caused primarily by forces transmitted through the buffer coating to the silica cladding of the fiber. These stresses are applied to the core, and if they are asymmetrical, will produce a change in the birefringence, which is the difference in the propagation constant of the light in each of the two characteristic polarizations. It is believed that the spatial frequency spectrum of the birefringence perturbations having a component at the beat length results in coupling of energy between the two modes. In coils, the undesired stresses and resultant coupling are often most intense where fibers cross each other. In addition, significant temperature dependencies can be caused by changes in the modulus of elasticity of the buffer coating (if it is a plastic-like material) and the differing thermal coefficients of expansion of the buffer and the fiber.
Typically Sagnac ring coils are wound on a form made of metal, plastic, or ceramic material. The objective is to provide a means of holding the coil in place when mounted in the mechanical package for the gyroscope, and to accurately locate the axis of the coil, as the fiber optic gyro is sensitive only to rotations about this axis. The coil form is typically either a cylinder or a flanged reel, and both introduce stresses, particularly as the temperature changes. Even at room temperature, the first layer of the coil often suffers from the forces associated with contact with the form, and the winding tension and pattern can have substantial effects on the h-parameter. Usually the coil form has a higher coefficient of thermal expansion than the fiber in the coil. Consequently, as the temperature increases, the radius of the form increases faster than that of the fiber, thereby increasing the radial pressure on the fiber. This results in a degradation in h. When the temperature decreases, the height of the flanged cylinder decreases, causing the flanges to press against the top and bottom surfaces of the coil. Encapsulation methods employing materials with a high Young's modulus, such as silicone rubber, also apply significant forces to the coil, due to the volumetric change during curing, and as a consequence of differential thermal expansion.